Hi everybody,

I am in the process of collecting all the parts for my Honda powered K20A (the Honda engine that spins the right way!) Locost. I have purchased an adapter plate that allows me to mate the K20A to a Mazda RX8 gearbox, the only thing is the weight of the gearbox 45kg!!. Does anybody know if the MX5/RX8 gearboxes are interchangeable?

Cheers

Hi there. The RX8 and MX5 6 speeds are basically the same AISIN AZ6 unit so they will weigh the same.
The ratios are the same too, apart from a slight difference in 4th. The gear lever in the MX5 box might be a little further back.
I have the IS200 variant in my car.

The Aisin 6 speed used by RX8 and mk3 mx5 is basically the same gearbox design but the main case is in two pieces the front half of which includes the bell housing which is different for RX and MX. Some later MX5's had different ratios for different markets.
The 5 speed mk3 MX box looks similar but there are no interchangeable parts of note IIRC and the MK1/2 MX box is totally different with regard to the gearbox casing as the power-plant frame mounts on the other side.

Aisin AZ6 (6 speed version) was use in RX8, IS200, Mk3 MX5, 200SX S15, BRZ, GT86.

Basically the same back end with all very similar (but not identical ratios). Not the 1st gear is pretty short (3.75 ish) but 2-6 are pretty much the same as a closer ratio Quaife Clubman but they are helical not straight cut. They should also be good for around 250hp and 250 ftlb as used in the 200SX but maybe more in a light car. Also good for 9k rpm as in the RX8 - at the price they are what's not to like if you can fit one in and get it mated up.

As said, front end and input shaft are vehicle specific noting the main difference with the RX8 and MX5 is not only the input shaft and bell housing bolts but in the MX5 the gearbox/engine are at a slight angle (7 degs comes to mind).

The main reason the RX8 is so popular is that the input shaft has the same splines as the old ford gearboxes (23 x 1" iirc) and so will fit the pinto clutch plate often used with Zetecs etc.

So, in short, RX8 and MX5 are not interchangeable as the bell housing and input shaft are different.